Aerogels are a unique class of ultra fine pore size, low density, high surface area, open-cell foams. Although many well-known methods of aerogel production are quite elaborate, in essence two fundamental steps are required. In the first step, precursor chemicals are added to a liquid solvent where they react to form a `gel,` which consists of a continuous, three-dimensional framework extending throughout the liquid solvent. In the second step, the solvent must be removed and replaced by air, leaving the dry solid framework, known as an aerogel. The second step is especially tricky because the force of the surface tension of the liquid solvent exceeds the strength of the tiny aerogel pores; therefore, if the solvent is removed by conventional techniques, the pores will be collapsed, destroying the desired aerogel product. Although aerogel pieces of extremely thin cross section have been made using evaporative methods of solvent removal, such methods cannot, to date, be used to obtain pieces greater than about 0.050 inch in thickness. See U.S. Pat. No. 5,420,168 issued May 30, 1995 to Mayer et al.
Through the manipulation of pressure and temperature, compounds can be transformed into `supercritical fluids`, which, among other interesting properties, have no surface tension. Relating this phenomena to aerogel production involves maintaining submersion of the solid framework until the liquid which fills the tiny pores is converted to a supercritical fluid, which is then extracted while in the supercritical state, thereby yielding an undamaged product.
There are many known methods of aerogel production; and, for the purposes of the present discussion, they may be divided into two method types. According to the first method, the precursor chemicals are mixed with a solvent, a gel forms and is brought to conditions where the pore liquid becomes supercritical, and then solvent extraction is carried out. According to the second method, the solvent in which the gel is formed is replaced with another solvent, which typically is not suitable to solvation of the precursor chemicals, but which has a supercritical temperature that is much lower than that of the original solvent.
In some cases, solvent exchange is performed because bringing the gel to conditions where the solvent becomes a supercritical fluid will damage the product, and the solvent must, therefore, first be replaced with a solvent which can be made supercritical without causing damage to the gel. In other cases, the original solvent could be supercritically extracted without damage to the gel, but the lower temperature extraction is more conducive to producing particular properties desired in the finished aerogel. In yet other cases, the original solvent is exchanged because the lower temperature needed to extract the final solvent is felt to be more desirable from a production standpoint, even though the solvent exchange step is very time consuming.
Bommel and Haan, in "Drying of Silica Gels with Supercritical Carbon Dioxide," J.Mater. Sci. 29 (1994) 943, report results of exchanging an alcohol solvent in a pilot carbon dioxide extraction apparatus. They found that the time required for exchanging ethanol with CO.sub.2 increases exponentially with gel thickness, and estimate that gels in the form of plates would require 1.5 hours for a 1 cm thick gel, 7 hours for a 2 cm thick gel, and 14 hours for a 3 cm thick gel.
U.S. Pat. No. 2,249,767, issued Jul. 22, 1941 to Kistler, describes the formation of silica-based gels by the reaction of sodium silicate and sulfuric acid in aqueous solution. After formation, the gel is washed to remove salts and excess acid and is placed in an autoclave, where the water is exchanged with alcohol. The alcohol is then supercritically extracted. The water is exchanged with alcohol because supercritical water will begin to re-dissolve the solid framework of the gel.
U.S. Pat. No. 3,672,833, issued Jun. 27, 1972 to Teichner et al., describes production of silica aerogel prepared by hydrolysis of either methyl or ethyl orthosilicate in alcohol; thereby, eliminating both the gel washing and solvent exchange steps. The alcohol-plus-water pore liquid was extracted supercritically at about 275.degree. C. without solvent exchange, but the product was limited to granular material, which was acceptable for its intended applications.
U.S. Pat. No. 4,327,065, issued Apr. 27, 1982, and U.S. Pat. No. 4,402,927, issued Sep. 6, 1983, both to von Dardel et al., teach a method of forming transparent silica aerogel monoliths. The described method requires that the gel be washed repeatedly with pure alcohol to remove the water portion of the alcohol and water pore liquid prior to supercritical extraction. After the gel is formed and washed, it is placed in an autoclave, where final treatment and extraction require at least 24 hours.
U.S. Pat. No. 4,432,956, issued Feb. 21, 1984, to Zarzycki et al. describes a process similar to above referenced U.S. Pat. No. 4,402,927, except the gels are not washed and much faster heating and venting of the autoclave is employed. Only relatively dense aerogels, however, could be produced by this method.
U.S. Pat. No. 4,610,863, issued Sep. 9, 1986 to Tewari et al., teaches the method of silica aerogel production that utilizes carbon dioxide as the extraction solvent.
U.S. Pat. No. 4,806,328, issued Feb. 21, 1989 to van Lierop et al., reports improved results relative to above referenced U.S. Pat. No. 4,432,956 by pre-pressurizing the autoclave to at least 50 bar prior to heating. This process also was able to produce fairly low-density aerogels.
U.S. Pat. Nos. 4,873,218, issued Oct. 10, 1989, and 4,997,804, issued Mar. 5, 1991, both to R. W. Pekala, describe some of the advantages and applications of organic aerogels, as well as teaching a method for producing a resorcinol-formaldehyde aerogel. As discussed in the patents, organic aerogels have distinct advantages over inorganic aerogels for many applications. U.S. Pat. Nos. 5,081,163, issued Jan. 14, 1992, and 5,086,085, issued Feb. 4, 1992, both also to R. W. Pekala, describe further applications of organic aerogels and teach preparation of transparent melamine-formaldehyde aerogels. The procedures described in the above four patents involve adding the precursor chemicals, plus a small amount of catalyst, to water in which the gel forms. The water is then exchanged with an organic solvent that is compatible with carbon dioxide. The organic solvent is then exchanged with liquid CO.sub.2, which is extracted somewhere above the relatively low (31.degree. C.) supercritical temperature of the CO.sub.2. The descriptions provided in the above patents illustrate the tedious and time-consuming nature of the solvent exchange steps.
U.S. Pat. Nos. 5,128,382, issued Jul. 7, 1992, and 5,252,620, issued Oct. 12, 1993, both to Elliott, Jr. et al., describe the formation of epoxy-based and methacrylate-based copolymer microcellular foams. The foam precursor chemicals were polymerized in either propane or freon-22 solvents, which were then supercritically extracted at about 100.degree. C. Several other solvents are listed which could be used, and it is noted that, "almost any solvent with a relatively low critical temperature (&lt;200.degree. C.) is a candidate." This patent also states without further elaboration that, "previous attempts to apply a similar process to resorcinol-formaldehyde aerogels resulted in substantial changes to the polymer product."
U.S. Pat. No. 5,275,796, issued Jan. 4, 1994 to Tillotson et al., describes a process for making silica aerogel monoliths that requires the preparation of a `condensed silica intermediate,` which becomes the precursor for a gel in which the primarily acetonitrile solvent is directly extracted supercritically at about 300.degree. C. and 2000 psig. The high temperatures and pressures produce an aerogel with a hydrophobic character which will hold up well when exposed to humidity, as opposed to silica aerogels produced by low temperature (i.e., CO.sub.2) extraction which have a hydrophilic character.
U.S. Pat. No. 5,484,818, issued Jan. 16, 1996 to De Vos et al., describes a variety of polyisocyanate-based organic aerogels, some of which were formed directly in solvents that were then extracted without solvent exchange.
U.S. Pat. Nos. 5,476,878, issued Dec. 19, 1995, 5,556,892, issued Sep. 17, 1996, and 5,744,510, issued Apr. 28, 1998, all to R. W. Pekala, describe further advantages and uses of organic aerogels, as well as teach the formation of a phenolic-furfural aerogel which has two distinct advantages over prior organic aerogels. First, the precursor chemicals are substantially less expensive than those used to produce prior-art organic aerogels. Second, phenolic-furfural gels are formed directly in an organic solvent that is compatible with carbon dioxide, thereby eliminating the tedious aqueous-to-organic solvent exchange. The solvents used were n-propanol and isopropanol, and it was noted that different solvents, such as ethanol or methanol, could also be used. The process described in these patents represents a significant improvement over the prior-art by eliminating the aqueous-to-organic solvent exchange step; however, the gel still must be flushed repeatedly with liquid CO.sub.2 to completely replace all of the organic solvent prior to supercritical extraction.
U.S. Pat. No. 5,686,031, issued Nov. 11, 1997 to Coronado et al., teaches a novel, highly efficient process for rapidly producing microporous materials, including aerogels. Prior to this process, the typical method for making aerogels consisted of forming gels in some sort of open mold, placing the mold in an autoclave, performing a solvent exchange if necessary, increasing the temperature and pressure to make the solvent supercritical, then venting the autoclave. As explained in the patent, a primary drawback of the autoclave method is that the rate of heating and venting must be very slow to avoid internal stresses and cracking of the gel. Because the pore size is so tiny, unless heating occurs very slowly, the liquid solvent within the gel expands faster than the solvent can flow through the pore matrix. The process described in the patent involves completely filling a closed metal mold with gel precursors, then heating rapidly, causing the internal temperature and pressure to rise above the critical points of the solvent. The gel's total confinement within a closed mold limits internal stresses, which allows the gel to be heated and vented extremely rapidly compared to autoclave processes. The patent reports that heating from room temperature to 300.degree. C. requires only about 15 minutes, and that venting from 200 bars to 2 bars also requires only about 15 minutes. The patent further reports that high temperature and pressure accelerate gel formation so extensively that silica gels are fully formed in the 15 minutes required for heating, which allows venting to begin as soon as supercritical conditions are reached. As stated in the patent, "[t]he present method allows rapid, semi-continuous processing of ultra fine pore materials for mass production, thereby making the manufacture of these types of materials cost-effective." Clearly, with the availability of this process, it is advantageous to eliminate the solvent exchange step whenever possible, unless it is absolutely necessary for production of a certain type of aerogel.
The above process, and the apparatus required to apply it, are also described by Poco et al., "A Rapid Supercritical Extraction Process for the Production of Silica Aerogels," Mat. Res. Soc. Symp. Proc., Vol. 431, 1996, 297-302. This article adopts the useful acronym, `RSCE,` standing for `Rapid SuperCritical Extraction,` and reports that, "the RSCE process can produce aerogels, from start-to-finish, 30 times faster than existing supercritical drying processes because the gel forms during the process and the liquids can be rapidly purged from the confined gel without cracking it."
Prior art organic aerogels have been shown to have numerous advantages and widespread potential applications. (For just a few examples of their potential, see U.S. Pat. Nos. 5,260,855, 5,601,938, and 5,698,140.) However, more rapid processing methods are needed in order that their manufacture be practicable for anything other than the most esoteric of applications. The present invention teaches methods by which mass-manufacture of organic aerogels becomes economically sensible.